Eukaryotic organisms are distinct from prokaryotes in possessing many intracellular organelle structures. Many of the metabolic reactions which separate eukaryotic biochemistry from prokaryotic biochemistry take place within these structures. In particular, many cellular functions require very strict reagent conditions, and the organelles enable compartmentalization and isolation of reactions which might otherwise cripple cytosolic metabolic processes.
Isolation of intracellular organelles from rat liver has demonstrated the presence of two distinct organelles, the lysosome and the peroxisome (de Duve, C. (1996) Ann. N.Y. Acad. Sci. 804:1-10). Lysosomes are the site of degradation of obsolete intracellular material during autophagy and of extracellular molecules following endocytosis and phagocytosis. They are derived from endosomes, which in turn are formed from budding of the trans-Golgi network (TGN) or from clathrin-coated membrane vesicles invaginating from the plasma membrane. Lysosomes contain hydrolytic enzymes, and the enveloping membranes of lysosomes and early/late endosomes are enriched in highly glycosylated transmembrane proteins of largely unknown function. Some lysosomal membrane proteins follow the constitutive secretory pathway and reach lysosomes indirectly via the cell surface. Other membrane proteins exit the TGN in clathrin-coated vesicles for direct delivery to endosomes and to lysosomes (Hunziker, W. and Geuze, H. J. (1996) BioEssays 18:379-389).
Genetic studies in yeast and biochemical studies in animal cells have provided evidence that the endocytic pathways and protein sorting in all eukaryotes probably share common enzymes and membrane components. An endocytic endosomal intermediate is responsible for the transport of the pheromone alpha-factor from the plasma membrane to the vacuole of the yeast, Saccharomyces cerevisiae. Proteins of the yeast endosomal membrane which may contribute to the transport of alpha-factor have been investigated in some detail. In particular, a protein with ten potential transmembrane domains, the EMP70 (p24a) precursor, has been identified (Singer-Kruger, B. et al. (1993) J. Biol. Chem. 268:14376-14386). Electron microscopic examination of yeast cells lacking functional EMP70 (p24a) shows a decrease in steady state vesicle accumulation and this suggests that EMP70 (p24a) is necessary for efficient vesicle budding (Stamnes, M. A. et al. (1995) Proc. Natl. Acad. Sci. 92:8011-8015). A similar protein, KIAA0255, has been identified in a human myoblast cell line (Nagese, T. et al. (1996) DNA Res. 3:321-329).
Protein sorting by transport vesicles, such as the endosome, has important consequences for a variety of physiological processes including cell surface growth, the biogenesis of distinct intracellular organelles, endocytosis, and the controlled release of hormones and neurotransmitters (Rothman, J. E. and Wieland, F. T. (1996) Science 272:227-234). In particular, neurodegenerative disorders and other neuronal pathologies are associated with biochemical flaws during endosomal protein sorting or endosomal biogenesis (Mayer R. J. et al. (1996) Adv. Exp. Med. Biol. 389:261-269).
The peroxisome is the site of many important metabolic reactions in eukaryotes such as lipid metabolism and gluconeogenesis, and is thought to cooperate intimately in biochemical reactions with the chloroplast (in plants and some protists) and the mitochondrion (in protists, animals, and plants). Peroxisomes are independent organelles and are not members of the secretory pathway family of organelles. They are characterized by a single membrane and a finely granulated matrix and are the site of many peroxide-generating oxidative reactions in the cell. Peroxisomes are unique among eukaryotic organelles in that their size, number, and enzyme content vary depending upon organism, cell type, and metabolic needs. Assembly of peroxisomes and their contents within the cell is termed biogenesis. Perixosome biogenesis can be divided into the following specific tasks: (1) membrane lipid acquisition, (2) proliferation/replication, (3) segregation, and (4) protein import. The majority of peroxisome-associated proteins are membrane-bound or are found proximal to the cytosolic or the lumenal side of the peroxisome membrane (Waterham, H. R. and Cregg, J. M. (1996) BioEssays 19:57-66).
Genetic defects in peroxisome proteins which result in peroxisomal deficiencies have been linked to a number of human pathologies, including Zellweger syndrome, rhizomelic chonrodysplasia punctata, X-linked adrenoleukodystrophy, acyl-CoA oxidase deficiency, bifunctional enzyme deficiency, classical Refsum's disease, DHAP alkyl transferase deficiency, and acatalasemia (Moser, H. W. and Moser, A. B. (1996) Ann. NY Acad. Sci. 804:427-441). Some of these peroxisome proteins are required for intracellular assembly of the organelle, including PAF-1, PXR1, and PXAA1 (Dodt, G. et al. (1996) Ann. N.Y. Acad. Sci. 804:516-523). Membrane protein homologs and their cDNA counterparts have been isolated from many organisms including the cyanobacterium Synechocystis (s111621), Candida boidinii (PMP20), and rat (peroxisomal 22 kDa membrane protein, PMP22) (Kaneko, T. et al. (1996) DNA Res. 3:109-136; Garrard, L. J. and Goodman, J. M. (1989) J. Biol. Chem. 264:13929-13937; and Kaldi, K. et al. (1993) FEBS Lett. 315:217-222). An mRNA which has some homology with peroxisome membrane proteins is downregulated in adenovirus 5-infected HeLa cells (DRAV5; Tomilin, N. and Doerfler, W. (1997) GenBank GI 1773069). Peroxisomal membrane proteins isolated from human liver include two integral membrane proteins of 22 kDa and 17 kDa (Santos, M. J. et al. (1994) J. Biol. Chem. 269:24890-24896). In addition, Gartner, J. et al. (1991; Pediatr. Res. 29:141-146) found a 22 kDa integral membrane protein associated with lower density peroxisome-like subcellular fractions in patients with Zellweger syndrome.
The discovery of three new human vesicle membrane protein-like proteins and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of developmental, vesicle trafficking, immunological, reproductive, and neoplastic disorders.